OPCIS-OPC Server Communication

An OPCIS can be configured to communicate with an OPC server either asynchronously or synchronously. Both communication methods have advantages and disadvantages.

• When communications are asynchronous, the OPCIS registers its points with the OPC server. When a point’s value changes, the OPC server pushes only the changed value to the OPCIS.

• When communications are synchronous, the OPCIS requests all data values at a specified “max update rate.”

Both communication methods have advantages and disadvantages.

Configuration	Advantage	Disadvantage
Asynchronous	Changes are sent as they occur. Only changed values are sent. Max push rate is configurable.	Not all OPC servers support asynchronous communication.
Synchronous	Max update rate is configurable.	All values sent with each update.

Recommended Communication Method

Asynchronous communication is the recommended method, when available.

Here is an example to illustrate why:

Say the update rate is 10 minutes.

• If the communications configuration is asynchronous you will get only the changed values every 10 minutes.
• If the communications configuration is synchronous you will get all values every 10 minutes. 

See OPC_IO_TYPE and OPC_MAX_UPDATE keywords for more information about configuration options for the OPCIS.